1. Field of the Invention
The present invention relates to an axle driving apparatus for driving a vehicle, and more particularly, the invention relates to a pair of displacement hydraulic systems where at least one of the systems is of a variable capacity type having a movable swash plate.
2. Description of Related Art
U.S. Pat. Nos. 4,914,907, 4,903,545, 5,156,576 and 5,201,692 disclose a type of axle driving apparatus, which houses in a common housing a hydrostatic transmission including a pair of displacement type hydraulic systems fluidly connected with each other and axles driven by the hydrostatic transmission. At least one of the displacement type hydraulic systems is of a variable displacement type having a movable swash plate in contact with heads of pistons fitted into a cylinder block. The pistons reciprocate and the movable swash plate slants with respect to the rotary axis of the cylinder block.
This type of movable swash plate is convex at a rear surface opposite to an abutting surface that abuts against the heads of the pistons. An inner surface of the housing facing the rear surface of the piston is made concave corresponding to the convex rear surface of the movable swash plate. A U-shaped thrust metal is positioned on the concave inner surface so that the rear surface of the swash plate is slantingly movable along a circular-arched guide surface of the thrust metal.
Since the movable swash plate is usually subjected to thrust from the piston, frictional resistance generated between the rear surface of the movable swash plate and the guide surface of thrust metal increases. Thus, the sliding resistance of the movable swash plate increases. Accordingly, a large force is necessary to operate the movable swash plate. Especially problematic is the case where the movable swash plate is spring biased to automatically return to a neutral position when an operation is stopped. In such case, when the sliding resistance is greater than the biasing force of the spring, the movable swash plate does not return to the neutral position. Further, the sliding resistance cannot be diminished merely by polishing the rear surface of movable swash plate, which contacts the thrust metal.